CN109541560B - Radiation calibration method based on communication signal tower - Google Patents
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Abstract
The invention provides a radiometric calibration method based on a communication signal tower, which comprises the following steps: establishing a simplified model of the communication signal tower: modeling a communication signal tower into a dihedral corner reflector and a cylindrical diffuser; calculating RCS of the communication signal tower of the dihedral corner reflector model; calculating RCS of a communication signal tower of the cylindrical diffuser model; calculating RCS of a permanent scatterer of the communication signal tower; calculating the target impulse response energy of each communication signal tower by using an integration method; solving a radiation calibration constant K; the invention researches the communication signal tower, considers the communication signal tower as a permanent scatterer in advance, simplifies the model of the permanent scatterer to calculate a signal tower model RCS, and performs radiation calibration by using the signal tower. The simulation verifies the accuracy, feasibility and effectiveness of the method and provides a new and effective way for realizing quantitative remote sensing in the global scope.
Description
Technical Field
The invention belongs to the field of engineering application, and particularly relates to a radiation calibration method based on a communication signal tower.
Background
In recent years, with the continuous transmission and lift-off of remote sensing satellites carrying high-resolution synthetic aperture radar (Synthetic Aperture Radar, SAR) systems, microwave remote sensing has been shifted from the qualitative remote sensing era to the quantitative remote sensing era. Scaling is an indispensable technical means for realizing quantitative earth observation of SAR imaging systems. Through SAR image radiometric calibration, the accurate relation between the spectral measurement and the ground object radar cross section (Radar Cross Section, RCS) or the backscattering coefficient can be constructed, and the SAR imaging system quantitative application is a precondition. Conventionally, radiometric calibration is generally performed by arranging a certain number of corner reflectors at an experimental site, calculating RCSs of the corner reflectors, and using a related radiometric calibration model. However, the arrangement of the corner reflectors is affected by many factors, so that the arrangement area is limited, and uniform arrangement in the global scope cannot be realized. Therefore, the SAR image radiometric calibration is realized by using various permanent scatterers (Permanent Scatterers, PS) to replace corner reflectors, and the SAR image radiometric calibration is an effective method. In general, PS can be defined as various ground object targets that are strong in the backscattering property to radar waves and stable in time series. According to the characteristics of PS, in the absence of a corner reflector, the corner reflector can be replaced by the PS to realize SAR radiation calibration. PS generally includes ground objects such as houses, buildings, etc., and has advantages of stable state, no time sequence and space limitation, and thus, unchanged scattering characteristics; the disadvantage is the complex structure and difficulty in accurately solving its RCS.
The correct and efficient calculation of the RCS of PS is a primary problem in SAR radiometric calibration. For example, in solving the RCS value of a regular corner reflector, some documents calculate the RCS of a dihedral corner reflector using a physical optical method and give characteristics of the dihedral corner RCS as a function of various parameters, discussing ways of changing its RCS. But this method is only applicable to dihedral corner reflectors and not to more complex diffusers. Some documents establish a general flow for realizing RCS estimation by using a physical optical method and a regional projection method, and calculate complete expressions of the RCS of the triangular corner reflectors in different incident directions. Although the above algorithm is simple to calculate, it is not suitable for calculating the RCS of a complex target. For complex targets, some documents introduce three types of RCS modeling schemes, namely a simple geometric body combination model method, a flat panel surface element method and a parameter surface model method, and give specific modeling modes and experiments of various methods. Some documents simulate RCS of a missile of a certain model by combining a fast far-field approximation theory and a multilayer fast multipole method, and although the calculation efficiency is improved, analysis modeling is not carried out on structural characteristics and scattering characteristics of a complex target.
Disclosure of Invention
Based on the technical problems, the invention provides a communication signal tower-based radiometric calibration method, which selects a communication signal tower to replace an angle diffuser, simulates the upper part of the communication signal tower into a dihedral angle diffuser according to the reflection characteristic and the structural characteristic of the angle diffuser, simulates the tower body into a cylinder, solves the RCS of the model by using a physical optical method, can express the RCS of the communication signal tower approximately, and uses the communication signal tower as a permanent diffuser (Permanent Scatterers, PS) to replace the angle diffuser to realize SAR image radiometric calibration, and has the following specific procedures:
step 1: establishing a simplified model of the communication signal tower: modeling a communication signal tower into a dihedral corner reflector and a cylindrical diffuser, wherein the specific steps comprise the following steps of 1.1 and 1.2:
step 1.1: modeling a metal plate at the top end of a communication signal tower and the upper surface of a trunk cylinder connected with the metal plate as a dihedral corner reflector, wherein the dihedral corner reflector comprises a surface 1 and a surface 2, the metal plate is the surface 1 of the dihedral corner reflector, the upper surface of the trunk cylinder connected with the metal plate is the surface 2, according to the structural characteristics of the signal tower, the metal plate at the top end of the signal tower is vertically connected with the trunk cylinder but is not completely connected with the trunk cylinder, when electromagnetic waves irradiate the signal tower, scattering occurs at the top end of the tower, so that the part of the top metal plate connected with the trunk cylinder is modeled as the dihedral corner reflector, wherein the dihedral corner reflector comprises the surface 1 and the surface 2, the widths of the dihedral corner reflector surface 1 and the dihedral corner reflector surface 2 are b, the height is h, the included angle between the surface 1 and the surface 2 is 2 beta, and the height of the surface 1 is overlapped with the height of the surface 2;
step 1.2: modeling a main cylinder of a communication signal tower as a cylindrical diffuser, wherein the main part of the communication signal tower is the most main diffuser, and the geometry of the main cylinder is a cylinder with relatively uniform thickness, so the main cylinder of the communication signal tower is modeled as the cylindrical diffuser, and the cylindrical diffuser has the following structure: the trunk cylinder is vertical to the ground, the lower surface of the trunk cylinder is intersected with the ground, the height of the trunk cylinder is assumed to be L, the trunk cylinder is divided into m independent small cylinders with the same height, and the height of each small cylinder is L, and l=L/m;
step 2: constructing a dihedral corner reflector model, and calculating the RCS of a communication signal tower of the dihedral corner reflector model, wherein the specific steps comprise the steps 2.1-2.4:
step 2.1: the RCS of the communication signal tower of the dihedral corner reflector model is expressed as: communication information of dihedral corner reflector model based on the sum of the reflected wave contributions of the surface 1 and the surface 2 and the mutual reflected wave contribution between the surface 1 and the surface 2RCS of the number tower is noted as: sigma (sigma) 1 ；
Wherein lambda is the radar incident wavelength, S 1 And S is 2 The reflected wave contributions for plane 1 and plane 2, respectively, S 3 And S is 4 Respectively contributing to the mutual reflection wave between the two planes;
step 2.2: calculation of the square root of RCS by physical optics is expressed as
Where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r A unit vector representing the polarization direction, v representing the unit vector of the magnetic field direction of the incident wave, f representing the position vector of the surface element of the scatterer, i and s representing the unit vectors of the incident direction and the scattering direction, respectively;
step 2.3: according to the formula (1) and the formula (2), S is calculated by a physical optical method 1 And S is 2 ：
Wherein, the incidence angle is assumed to be theta, and beta is more than or equal to theta and less than or equal to beta, and beta is half of the included angle between the surface 1 and the surface 2; wherein b 1 And b 2 The reflected wave widths of the surfaces 1 and 2, respectively, c is the speed of light in vacuum, and the contribution S to the inter-surface reflected wave 3 And S is 4 Because the incident wave is not completely irradiated on two planes due to the problem of the direction of the incident wave, S is obtained by the ray tracing method 3 And S is 4 Is a true illumination width b of (2) 3 And b 4 ：
Step 2.4: s is obtained by calculation by a ray tracing method according to the formula (1) and the formula (2) 3 And S is 4 Corresponding real illumination width b 3 And b 4 Further calculate S 3 And S is 4 ：
Wherein, alpha=pi-3 beta,
thereby obtaining the mutual reflection wave contribution S between the two faces of the corner reflector 3 And S is 4 The method comprises the following steps:
step 3: constructing a cylindrical diffuser model, and calculating RCS of a communication signal tower based on the cylindrical diffuser model, wherein the specific flow is as follows:
step 3.1: according to the cylindrical diffuser structure, a polar coordinate system is built in a small cylinder
Taking any point B on the surface of the small cylinder, taking the center point of the round surface where the point B is located as the origin of coordinates (0, 0), and describing an original coordinate system (x, y, z) as follows: to sit onThe vertical direction of the coordinate origin is the positive coordinate axis of the z axis, the horizontal direction of the coordinate origin is the positive coordinate axis of the y axis to the left, the direction vertical to both the y axis and the x axis is the x axis, and the positive direction of the x axis is outwards; polar coordinate system in small cylinder>
The description is as follows: the distance between the point B and the origin is r axis, and the positive included angle between the distance between the point B and the origin and the x axis is +.>
The z axis in the polar coordinate system coincides with the z axis of the original coordinate system;
step 3.2: defining a surface position vector p;
the surface elements of the small cylinder are expressed as:
wherein dW is any point element on the surface of the cylinder, a is the radius of the cylinder, and the surface position vector p is expressed as:
p＝p 0 +zZ+an (10)
wherein Z is a unit vector along the axial direction of the cylinder, and p 0 For a position vector from a certain point to a cylinder center point, n represents an external normal vector of the surface;
step 3.3: the RCS of the communication signal tower based on the cylindrical diffuser model is calculated, and the flow is as follows:
substituting equations (9) and (10) into equation (2), the RCS square root physical optical expression can be expressed as:
where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r Represents the unit vector of the polarization direction, v represents the unit vector of the magnetic field direction of the incident waveI and s represent unit vectors of the incident direction and the scattering direction, respectively;
and equation (11) can be expressed as an integral variable in the axial and circumferential directions:
wherein I is z And
the method comprises the following steps of:
wherein I is z And
the integral variables in the axial direction and the circumferential direction are respectively, l is the height of a small cylinder, and the integral I in the axial direction is z The calculation of (2) can be expressed as:
circumferential integration using stationary phase method
Can be expressed as:
substituting equations (15) and (16) into equation (12), equation (12) can be simplified to:
wherein n is 0 Is an outward surface normal to the axis. Squaring the two ends of the formula (17) to obtain a small cylinder with RCS of sigma 2 ：
Wherein, the free space wave number k=2pi/lambda, and the RCS of m small cylinders are cumulatively summed to obtain the RCS value m x sigma of the whole cylinder 2 Namely the RCS of the communication signal tower of the cylindrical diffuser model;
step 4: calculating the RCS of the whole communication signal tower:
σ＝σ 1 +m×σ 2 (19)
wherein sigma is RCS, sigma of the whole communication signal tower 1 RCS, mxSigma of communication signal tower as dihedral corner reflector model 2 RCS of communication signal tower of cylindrical diffuser model;
step 5: calculating the target impulse response energy of each communication signal tower by an integration method: the method comprises the steps of respectively extracting the positions of a plurality of communication signal towers in a scene SAR image, assuming that the signal towers are identical in specification, respectively intercepting pixel areas in an intensity image according to intensity, and calculating target impulse response energy epsilon of each communication signal tower by an integration method, wherein the expression is as follows:
wherein DN is i N is the intensity value of the pixel in the image A N is the number of pixels of the integral area of the signal tower in the image B The number of pixels, delta, is the uniform area around the signal tower a And delta r The azimuth and distance pixel spacing, respectively.
Step 6: solving the radiation calibration constant K,
wherein sigma is the RCS value of the whole communication signal tower, and theta is the radar incident angle. In the space-borne SAR image radiometric calibration, the reflection characteristics of the actual target represented by each pixel in the SAR image are required to be precisely calibrated. The correspondence between the reflection characteristics of the object and the pixel values on its corresponding image can be represented by a transfer function. For a stable SAR imaging system, the transfer function is a constant, which is called a radiometric scaling constant K, so that the invention mainly solves the average radiometric scaling constant of a scene image. PS appears as a strongly reflecting spot on the image and can therefore be regarded as a spot target with known scattering properties.
The beneficial technical effects are as follows:
aiming at the problem that the arrangement of the corner reflectors in radiometric calibration is limited by various conditions, the invention provides a method for realizing radiometric calibration by using the mobile communication signal tower as a permanent scatterer instead of the corner reflectors, which well solves the problem and achieves good technical effects. Traditionally, the calibration is finished by arranging a series of corner reflectors for point target calibration or by large-scale distribution targets such as tropical rain forest, but the two methods have a certain degree of defects: the distribution conditions of the point targets are limited, and the distribution targets are difficult to find. In order to solve the problems that the traditional radiometric calibration method based on corner reflectors and the like is limited in layout and insufficient in obtaining ground surface information, the invention researches the communication signal tower, considers the communication signal tower as a permanent scatterer in advance, simplifies the model of the communication signal tower to calculate a signal tower model RCS, and performs radiometric calibration by using the signal tower. The invention fundamentally changes the radiation calibration method, breaks through the traditional thought that only corner reflectors and tropical rain forests are manually arranged as calibration devices, promotes the rapid development of the radiation calibration technology, and provides a new and effective way for realizing quantitative remote sensing in the global scope.
The invention provides two main research aspects aiming at a communication signal tower: firstly, a signal tower is taken as a permanent scatterer, an RCS model is established, and an RCS is solved by using a physical optical method; and secondly, performing radiation scaling treatment by taking the solved signal tower as a new scaler. The method not only realizes the establishment of a simplified model of the signal tower and the rapid quasi-solution of the RCS value of the signal tower, but also realizes the invention of using the signal tower as a new scaler, and the accuracy, feasibility and effectiveness of the method are verified by simulation.
Drawings
FIG. 1 is a flow chart of a radiometric calibration method based on a communication signal tower according to an embodiment of the invention;
FIG. 2 is a simplified model of a communication signal tower according to an embodiment of the present invention;
FIG. 2 (a) is a three-dimensional structure of a communication signal tower;
FIG. 2 (b) is a simplified model of a communication tower;
FIG. 3 is a dihedral angle emitter model in an embodiment of the present invention;
FIG. 4 is a diagram of a cylindrical scatterer model in an embodiment of the invention;
FIG. 5 is a graph showing the result of the calculation of RCS by the FEKO software in accordance with an embodiment of the present invention;
FIG. 5 (a) shows the RCS result of a communication signal tower based on dihedral corner reflector model calculated by FEKO software;
FIG. 5 (b) is the RCS effect of a communication signal tower based on a cylindrical diffuser model;
FIG. 6 is a high-resolution SAR image of Funew city region III in accordance with the present invention;
FIG. 7 is a graph of tower intensity from 1 to 8 points in an embodiment of the present invention;
in the figure: 1-a metal plate at the top end of a signal tower; the 2-signal tower is connected with the upper surface of the trunk column of the metal plate; 3-a trunk cylinder; 4-small cylinders.
Detailed Description
The invention is further described with reference to the accompanying drawings and specific examples, and experimental environment is as follows: model solving is achieved on a system with a CPU of Core (TM) i 5-3470.3.20 GHz, a memory of 8GB and a Windows7 flagship version by using MATLAB7.12.0 software programming, and the model is solved by FEKO7.0 software. The invention provides a radiation calibration method based on a communication signal tower, which is shown in fig. 1, and comprises the following specific procedures:
step 1: establishing a simplified model of the communication signal tower: modeling a communication signal tower into a dihedral corner reflector and a cylindrical diffuser, as shown in fig. 2, wherein fig. 2 (a) is a three-dimensional structure of the communication signal tower; fig. 2 (b) is a simplified model of a communication signal tower, and the specific steps include steps 1.1 and 1.2:
step 1.1: modeling a metal plate 1 at the top end of a communication signal tower and the upper surface 2 of a trunk cylinder connected with the metal plate as a dihedral corner reflector, wherein the dihedral corner reflector comprises a face 1 and a face 2, the metal plate 1 is the face 1 of the dihedral corner reflector, the upper surface 2 of the trunk cylinder connected with the metal plate is the face 2, according to the structural characteristics of the signal tower, the metal plate at the top end of the signal tower is vertically connected with the trunk cylinder but is not completely butted, when electromagnetic waves irradiate the signal tower, scattering occurs at the top end of the tower, so that the part of the top metal plate connected with the trunk cylinder is modeled as the dihedral corner reflector, wherein the dihedral corner reflector comprises the face 1 and the face 2, the widths of the dihedral corner reflector face 1 and the face 2 are b, the height is h, the included angle between the face 1 and the face 2 is 2 beta, and the height of the face 1 coincides with the height of the face 2, as shown in fig. 3;
step 1.2: modeling a main cylinder 3 of a communication signal tower as a cylindrical diffuser, wherein the main part of the communication signal tower is the most main diffuser, the geometry of the main cylinder is a cylinder 3 with relatively uniform thickness, and therefore, the main cylinder of the communication signal tower is modeled as the cylindrical diffuser, and the cylindrical diffuser has the following structure: the trunk cylinder 3 is vertical to the ground, the lower surface of the trunk cylinder is intersected with the ground, the trunk cylinder is divided into m independent small cylinders 4 with equal height, the height of each small cylinder 4 is L, and l=l/m, as shown in fig. 4;
in order to verify the effectiveness of the algorithm, the PS model is selected from the experiment, wherein the height is 40m, the diameter of the top end is 0.5m, the diameter of the bottom end is 1m, the material is a metal cylinder, and part of signal towers distributed in Funew city areas are PS models. The experiment was performed in the case where the specifications of each signal tower were almost the same, regardless of the problems of the external conditions such as the difference in the topography of each signal tower. The settings of the relevant parameter values of the dihedral corner reflectors and the cylinders simulated in the experiment are shown in table 1, and the given parameter values are obtained from empirical values. b and h respectively represent the width and the height of the dihedral corner reflector, beta is the included angle of the dihedral corner reflector, and a and l respectively represent the radius and the height of the small cylinder.
TABLE 1 parameter settings
Step 2: constructing a dihedral corner reflector model, and calculating the RCS of a communication signal tower of the dihedral corner reflector model, wherein the specific steps comprise the steps 2.1-2.4:
step 2.1: the RCS of the communication signal tower of the dihedral corner reflector model is expressed as: based on the sum of the reflected wave contributions of the surfaces 1 and 2 and the mutual reflected wave contribution between the surfaces 1 and 2, the RCS of the communication signal tower of the dihedral corner reflector model is written as: sigma (sigma) 1 ；
Wherein lambda is the radar incident wavelength, S 1 And S is 2 The reflected wave contributions for plane 1 and plane 2, respectively, S 3 And S is 4 Respectively contributing to the mutual reflection wave between the two planes;
step 2.2: calculation of the square root of RCS by physical optics is expressed as
Where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r Represents the unit vector of polarization direction, v represents incidenceThe magnetic field direction unit vector of the wave, f represents the position vector of the surface element of the scattering body, and i and s represent the unit vectors of the incident direction and the scattering direction respectively;
step 2.3: according to the formula (1) and the formula (2), S is calculated by a physical optical method 1 And S is 2 ：
Wherein, the incidence angle is assumed to be theta, and beta is more than or equal to theta and less than or equal to beta, and beta is half of the included angle between the surface 1 and the surface 2; wherein b 1 And b 2 The reflected wave widths of the surfaces 1 and 2, respectively, c is the speed of light in vacuum, and the contribution S to the inter-surface reflected wave 3 And S is 4 Because the incident wave is not completely irradiated on two planes due to the problem of the direction of the incident wave, S is obtained by the ray tracing method 3 And S is 4 Is a true illumination width b of (2) 3 And b 4 ：
Step 2.4: s is obtained by calculation by a ray tracing method according to the formula (1) and the formula (2) 3 And S is 4 Corresponding real illumination width b 3 And b 4 Further calculate S 3 And S is 4 ：
Wherein, alpha=pi-3 beta,
thereby obtaining the mutual reflection wave contribution S between the two faces of the corner reflector 3 And S is 4 The method comprises the following steps:
step 3: constructing a cylindrical diffuser model, and calculating RCS of a communication signal tower based on the cylindrical diffuser model, wherein the specific flow is as follows:
step 3.1: according to the cylindrical diffuser structure, a polar coordinate system is built in a small cylinder
Taking any point B on the surface of the small cylinder, taking the center point of the round surface where the point B is located as the origin (0, 0) of coordinates, and describing an original coordinate system (x, y, z) as follows: taking the vertical upward direction of the origin as a positive coordinate axis of a z axis, taking the horizontal leftward direction of the origin as a positive coordinate axis of a y axis, taking the direction vertical to both the y axis and the x axis as an x axis, and enabling the x axis to be positive outwards; polar coordinate system in small cylinder>
The description is as follows: the distance between the point B and the origin is r axis, and the positive included angle between the distance between the point B and the origin and the x axis is +.>
The z-axis in the polar coordinate system coincides with the z-axis of the original coordinate system as shown in fig. 4;
step 3.2: defining a surface position vector p;
the surface elements of the small cylinder are expressed as:
wherein dW is any point element on the surface of the cylinder, a is the radius of the cylinder, and the surface position vector p is expressed as:
p＝p 0 +zZ+an (10)
wherein Z is a unit vector along the axial direction of the cylinder, and p 0 For a position vector from a certain point to a cylinder center point, n represents an external normal vector of the surface;
step 3.3: the RCS of the communication signal tower based on the cylindrical diffuser model is calculated, and the flow is as follows:
substituting equations (9) and (10) into equation (2), the RCS square root physical optical expression becomes:
where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r A unit vector representing the polarization direction, v represents a unit vector of the magnetic field direction of the incident wave, and i and s represent unit vectors of the incident direction and the scattering direction, respectively;
and equation (11) can be expressed as an integral variable in the axial and circumferential directions:
wherein I is z And
the method comprises the following steps of:
wherein I is z And
the integral variables in the axial direction and the circumferential direction are respectively, l is the height of a small cylinder, and the integral I in the axial direction is z The calculation of (2) can be expressed as:
circumferential integration using stationary phase method
Can be expressed as:
substituting equations (15) and (16) into equation (12), equation (12) can be simplified to:
squaring the two ends of the formula (17) to obtain a small cylinder with RCS of sigma 2 ：
Wherein the wave number k=2pi/lambda, and cumulatively summing the RCS of the m small cylinders to obtain the RCS value m x sigma of the whole cylinder 2 Namely the RCS of the communication signal tower of the cylindrical diffuser model;
step 4: calculating the RCS of the whole communication signal tower:
σ＝σ 1 +m×σ 2 (19)
wherein sigma is RCS, sigma of the whole communication signal tower 1 RCS, mxSigma of communication signal tower as dihedral corner reflector model 2 RCS of communication signal tower of cylindrical diffuser model;
substituting the parameters listed in Table 1 into formulas (3), (4), (7) and (8) to calculate the total RCS value of the four-side scattered echo contribution of the dihedral corner reflector to be-134.3 db sm, and calculating the RCS value of the dihedral corner reflector from formula (1) to be 14.35db sm. The RCS value of each small cylinder was calculated to be 0.74db sm using equation (18), and then the 40 small cylinders were summed to give an RCS value of 29.60db sm for the whole cylinder. Thus, each signal tower RCS solved by the algorithm of the present invention is approximately 43.95dbsm from equation (19).
In order to quantitatively evaluate the experimental result of the algorithm, the validity and accuracy of the algorithm are verified by taking the result of the RCS theoretical formula and the result of the FEKO7.0 software calculation as examples. The RCS theoretical formula is respectively:
wherein sigma d Is the theoretical value of dihedral angle RCS, sigma c Is the theoretical value of the RCS of the cylinder,
for azimuth angle, in the present invention +.>
Wherein the calculation result using the REKO software is shown in fig. 5 (a) and (b).
To visually compare the results of the three algorithms, they are listed in table 2:
table 2 experimental results of three algorithms
From comparison of three results, the algorithm of the invention is used for respectively carrying out experiments on the dihedral corner reflectors and the cylindrical type, so that the relative deviation of the RCS of the signal tower is respectively 5.38 and 0.44db sm, the minimum precision is more than 89%, and the maximum precision is 99%, thus obtaining a better solving result. In actual calculation, the FEKO is used for solving the signal tower model, so that the memory occupied by the signal tower model is large, the time consumption is long, and compared with the traditional algorithm, the algorithm is more convenient and faster. According to analysis, the simulated dihedral angles and the cylinders are independent simple models and are different from the complex structure of the signal tower, such as two circles of metal covers on the upper part of the signal tower, so that the obtained result is slightly smaller than that calculated by FEKO software. Meanwhile, due to the influence of conditions such as an incident angle, a propagation error and the like, the result is quite different from the result obtained by a theoretical formula. The key of the algorithm is to build a model of the signal tower, simplify the signal tower into a simple shape and a corner reflector model, and overcome the defects of uneven layout and limited places when the corner reflectors are calibrated. Therefore, the RCS solved by the algorithm, the RCS theoretical formula and the traditional FEKO software of the invention has certain errors, but can be ignored in practical application engineering.
Step 5: calculating the target impulse response energy of each communication signal tower by an integration method: the method comprises the steps of respectively extracting the positions of a plurality of communication signal towers in a scene SAR image, assuming that the signal towers are identical in specification, respectively intercepting pixel areas in an intensity image according to intensity, and calculating target impulse response energy epsilon of each communication signal tower by an integration method, wherein the expression is as follows:
wherein DN is i N is the intensity value of the pixel in the image A N is the number of pixels of the integral area of the signal tower in the image B The number of pixels, delta, is the uniform area around the signal tower a And delta r The azimuth and distance pixel spacing, respectively.
Step 6: solving the radiation calibration constant K,
wherein sigma is the RCS value of the whole communication signal tower, and theta is the radar incident angle. In the space-borne SAR image radiometric calibration, the reflection characteristics of the actual target represented by each pixel in the SAR image are required to be precisely calibrated. The correspondence between the reflection characteristics of the object and the pixel values on its corresponding image can be represented by a transfer function. For a stable SAR imaging system, the transfer function is a constant, which is called a radiometric scaling constant K, so that the invention mainly solves the average radiometric scaling constant of a scene image. PS appears as a strongly reflecting spot on the image and can therefore be regarded as a spot target with known scattering properties.
The experimental data shown in fig. 6 is a scene GF-3SAR image covering the region of Funew city, the imaging time is 2017, 11, 15 days, the imaging range is 10×10km, the specific parameters are shown in table 3, the points 1 to 18 in fig. 6 are 18 signal towers with almost the same specifications, and the distribution area is in the region of Funew city.
TABLE 3GF-3 image parameters
Meanwhile, from 1 to 18 signal towers are selected as research objects, and the signal towers 1 to 8 are taken as examples, and the intensity images of the signal towers are shown in fig. 7. However, the signal towers No. 8, no. 10 and No. 14 have larger errors in the integral areas of the signal towers in the intensity images due to the influence of nearby objects, so that the 3 signal towers are removed; since the tower specifications are chosen to be nearly identical, the present invention assumes that the RCS for each tower is identical. However, the distribution area of the signal towers is different from the corner reflectors, the signal towers are generally distributed in an open area, the distribution of the signal towers is irregular, the response energy is greatly different, the adaptive integration areas are intercepted respectively according to the intensity image of each signal tower, and the response energy of each signal tower is solved.
The response energy epsilon of PS in the image is calculated by using the formula (19), the radiometric calibration constant K is solved by using the formula (20), and compared with the nominal radiometric calibration constant in the GF-3SAR image metadata, and the experimental result is shown in the table 4.
As shown in the table, under the condition that each signal tower RCS is the same, the average value of the response energy of 15 signal towers is solved by using an integration method, so that the radiation calibration constant of the experimental image is 23.19db, and compared with the nominal radiation calibration constant, the difference value of the two is 0.41db, and the radiation calibration requirement of 1db is met, thereby obtaining a better calibration result.
TABLE 4 radiation calibration results
Claims (1)
1. The radiation calibration method based on the communication signal tower is characterized by comprising the following steps of:
step 1: establishing a simplified model of the communication signal tower: modeling a communication signal tower into a dihedral corner reflector and a cylindrical diffuser, wherein the specific steps comprise the following steps of 1.1 and 1.2:
step 1.1: modeling a metal plate at the top end of a communication signal tower and the upper surface of a trunk cylinder connected with the metal plate into a dihedral corner reflector, wherein the dihedral corner reflector comprises a surface 1 and a surface 2, the metal plate is the surface 1 of the dihedral corner reflector, the upper surface of the trunk cylinder connected with the metal plate is the surface 2, the widths of the surface 1 and the surface 2 are b, the height is h, the included angle between the surface 1 and the surface 2 is 2β, and the height of the surface 1 is overlapped with the height of the surface 2;
step 1.2: modeling a main cylinder of a communication signal tower as a cylindrical diffuser, wherein the cylindrical diffuser has the structure that: the trunk cylinder is vertical to the ground, the lower surface of the trunk cylinder is intersected with the ground, the height of the trunk cylinder is assumed to be L, the trunk cylinder is divided into m independent small cylinders with the same height, and the height of each small cylinder is L, and l=L/m;
step 2: constructing a dihedral corner reflector model, and calculating the RCS of a communication signal tower of the dihedral corner reflector model, wherein the specific steps comprise the steps 2.1-2.4:
step 2.1: the RCS of the communication signal tower of the dihedral corner reflector model is expressed as: based on the sum of the reflected wave contributions of the surfaces 1 and 2 and the mutual reflected wave contribution between the surfaces 1 and 2, the RCS of the communication signal tower of the dihedral corner reflector model is written as: sigma (sigma) 1 ；
Wherein lambda is the radar incident wavelength, S 1 And S is 2 The reflected wave contributions for plane 1 and plane 2, respectively, S 3 And S is 4 Respectively contributing to the mutual reflection wave between the two planes;
step 2.2: calculation of the square root of RCS by physical optics is expressed as
Where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r A unit vector representing the polarization direction, v representing the unit vector of the magnetic field direction of the incident wave, f representing the position vector of the surface element of the scatterer, i and s representing the unit vectors of the incident direction and the scattering direction, respectively;
step 2.3: according to the formula (1) and the formula (2), S is calculated by a physical optical method 1 And S is 2 ：
Wherein, the incidence angle is assumed to be theta, and beta is more than or equal to theta and less than or equal to beta, and beta is half of the included angle between the surface 1 and the surface 2; wherein b 1 And b 2 The reflected wave widths of the surface 1 and the surface 2 are respectively, and c is the speed of light in vacuum;
step 2.4: s is obtained by calculation by a ray tracing method according to the formula (1) and the formula (2) 3 And S is 4 Corresponding real illumination width b 3 And b 4 Further calculate S 3 And S is 4 ：
Wherein, alpha=pi-3 beta,
thereby obtaining the mutual reflection wave contribution S between the two faces of the corner reflector 3 And S is 4 The method comprises the following steps:
wherein c is the speed of light in vacuum;
step 3: constructing a cylindrical diffuser model, and calculating the RCS of a communication signal tower of the cylindrical diffuser model, wherein the specific steps comprise the steps of 3.1-3.3:
step 3.1: according to the cylindrical diffuser structure, a polar coordinate system is built in a small cylinder
Step 3.2: defining a surface position vector p;
the surface elements of the small cylinder are expressed as:
wherein dW is any point element on the surface of the cylinder, a is the radius of the cylinder, and the surface position vector p is expressed as:
p＝p 0 +zZ+an (10)
wherein Z is a unit vector along the axial direction of the cylinder, and p 0 For a position vector from a certain point to a cylinder center point, n represents an external normal vector of the surface;
step 3.3: the RCS of the communication signal tower of the cylindrical diffuser model is calculated, and the flow is as follows:
substituting equations (9) and (10) into equation (2), the RCS square root physical optical expression becomes:
where j represents a complex number with a modulus of 1, k represents the free space wavenumber, k=2pi/λ, t represents the illuminated area of the diffuser, n represents the external normal vector of the surface, e r A unit vector representing the polarization direction, v represents a unit vector of the magnetic field direction of the incident wave, and i and s represent unit vectors of the incident direction and the scattering direction, respectively;
and expression (11) is expressed as an integral variable in the axial and circumferential directions:
wherein I is z And
the method comprises the following steps of:
wherein I is z And
the integral variables in the axial direction and the circumferential direction are respectively, l is the height of a small cylinder, and the integral I in the axial direction is z The calculation of (2) is expressed as:
circumferential integration using stationary phase method
Expressed as:
substituting equations (15) and (16) into equation (12), equation (12) is simplified to:
wherein n is 0 Squaring the two ends of equation (17) to obtainRCS to small cylinder is sigma 2 ：
Wherein, the free space wave number k=2pi/lambda, and the RCS of m small cylinders are cumulatively summed to obtain the RCS value m x sigma of the whole cylinder 2 Namely the RCS of the communication signal tower of the cylindrical diffuser model;
step 4: calculating the RCS of the whole communication signal tower:
σ＝σ 1 +m×σ 2 (19)
wherein sigma is RCS, sigma of the whole communication signal tower 1 RCS, mxSigma of communication signal tower as dihedral corner reflector model 2 RCS of communication signal tower of cylindrical diffuser model;
step 5: calculating the target impulse response energy of each communication signal tower by using an integration method, wherein the expression is as follows:
wherein DN is i N is the intensity value of the pixel in the image A N is the number of pixels of the integral area of the signal tower in the image B The number of pixels, delta, is the uniform area around the signal tower a And delta r The pixel intervals are respectively azimuth and distance;
step 6: solving a radiation scaling constant K:
wherein sigma is RCS of the whole communication signal tower, and theta is radar incident angle.
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